The present invention is directed to a method for determining the shielding effect of a shielded, particularly permanently installed cabling path.
Different line types are available in current technology for the various usable frequency ranges for line-bound transmission of electromagnetic signals. Lines and cables with unshielded copper wires are mainly employed in direct current and low-frequency technology. Due to the high-frequency effects that increasingly occur (skin effect, noise emission, high attenuation, reflections, etc.), such cables are no longer suitable as transmission paths for the frequency ranges used for communications and high-frequency technology. Instead, and in addition to modern light waveguides (for example, optical fiber lines), it is especially shielded cables or, respectively, wave guides (symmetrical cables and coaxial cables) that are utilized wherein a line wire (referred to below as inside conductor) is surrounded by an envelope of conductive material formed, for example, as a wire weave over an insulation layer or dielectric. A signal supplied into such a cable propagates as electromagnetic wave in the interspace of inside conductor and shielding, whereby the disturbing influences of the high-frequency effects are largely avoided, particularly the emission of energy into the space surrounding the cable.
In practice, such shielded cables are utilized, for example, in the framework of an information-technology building cabling. After the installation of the cable, measurements of the operating behavior of this transmission links according to applicable standards (for example, EN 50173) are required, particularly with respect to the electromagnetic compatibility (EMC).
Electromagnetic fields no longer occur outside an ideal coaxial shielding (or outside a concentric shielding given multi-lead cables). A real shielding, however, exhibits ohmic and inductive resistances that, when combined, are referred to as its surface transfer impedance. The quality of a shielding is thereby usually defined with reference to its surface transfer impedance. The shielding effect that can be achieved increases with lower surface transfer impedance of a shielding. The individual components, such as cable and plug connectors, of a cabling path are usually fabricated using defined measuring methods. During installation of a cabling path, however, mistakes can occur in that, for example, cable shieldings are not carefully connected to plug connectors or parts of a shielding are damaged.
The methods currently applied in practice for testing the shielding quality of an installed and, therefore usually poorly accessible cabling path are usually limited to the measurement of the D.C. resistance of the shielding, whereby frequency-dependent effects such as, for example, the reduction of the shielding effect due to inductively acting shield connections ("pig tails") cannot be detected. In addition, what are referred to as "sniffer probes" are also utilized with which radiation emerging from the shielding can be sought. A dependable and quantitative determination of the shielding quality, however, is not possible therewith.